Lead chromate pigments, which contain lead chromate as major component, include chrome yellow pigments which provide hues ranging from a greenish yellow to orange and molybdate red or orange pigments containing lead chromate-molybdate which provides hues ranging from orange to red, as specified in the Japanese Industrial Standard (JIS K5110), International Organization for Standardization (ISO 3711), American Society for Testing and Materials (ASTM 211), and Deutsche Industrie Normen (DIN 55975). Lead chromate pigments are represented by Colour Index (CI) Nos. 77600, 77601, 77603, and 77605.
Lead chromate pigments are excellent pigments characterized by their brilliance of tone and high hiding power, and are used in paints and as colorants for synthetic resins. These pigments are highly suitable for use in public applications because they are indispensable, in particular, as a traffic paint material.
However, lead chromate pigments have a drawback in that their brilliant tones are apt to fade because they readily undergo chemical changes upon contact with acids, alkalis, or sulfides, such as hydrogen sulfide, in the air.
Further, lead chromate pigments suffer fading when an external stimulus such as heat or ultraviolet rays is applied thereto, as such a stimulus causes the pigment compositions to release part of the oxygen contained therein and to reduce hexavalent chromium. For this reason, if such lead chromate pigments are used with synthetic resins such as polyethylene, polypropylene, ABS and polyamide resins, color fading of the pigments is unavoidable because these resins are molded at temperatures not lower than 200.degree. C.
Conventionally, a number of measures have been proposed and practiced for suppressing the color fading of lead chromate pigments to thereby improve their performance.
For example, U.S. Pat. Nos. 2,212,917, 2,296,638, 2,316,244 and 2,365,171 disclose the use of water-soluble aluminum salts, titanium salts, zirconium salts, silicates, antimony salts, cerium salts, etc., as treating materials for lead chromate pigments.
Improvements in the above approaches have recently been proposed which involve coating the lead chromate pigments with silica. Such a coating approach is the most commonly used technique at present. See JP-B-46-9555, JP-B-46-34788, JP-B-46-42713, JP-B-48-32415, JP-B-49-20925 and JP-B-50-14254. The term "JP-B" as used herein means an "examined Japanese patent publication".
In addition to the silica-treatment technique, it has also been proposed to suppress the color fading of lead chromate pigments by the mixed use of silica and antimony. See U.S. Pat. No. 3,690,906.
It is also known that lead chromate pigments can be treated with a metal oxide derived from a water-soluble metal compound by heating the lead chromate pigments in hot water under pressure while stirring, thereby attaining improved heat resistance, light resistance, etc. See JP-B-49-16531.
Of the large number of proposals for improvements in the color fastness properties of lead chromate pigments, the silica-coated lead chromate pigments, which are obtained by densely coating the pigment particles with amorphous silica, are currently regarded as most reliable.
However, since the conditions under which lead chromate pigments are used have become more and more severe in recent years, even the conventional lead chromate pigments coated with silica, silica-alumina or zirconium-silica are still unsatisfactory with respect to meeting quality requirements regarding improvements in color fastness properties and with respect to the suppression of the dissolution of heavy metal components.
One reason for the above may be that slight amounts of chromium and lead which dissolve out during the silica-coating reaction are reprecipitated or adsorbed, causing the resulting silica coating to be incomplete.
Further, the lead chromate pigments of U.S. Pat. No. 3,690,906 which have been treated with silica-antimony are significantly inferior to the silica-coated lead chromate pigments in color fastness properties and in the suppression of the dissolution of heavy metal components. This inferiority is due to a deficient amount of silica used as well as the presence of a large number of translucent gel-like particles that are in a free state without forming a coating, the presence of these particles being attributable to the use of improper conditions for silica formation. Their presence can be ascertained by examination with a transmission electron microscope (at a magnification of 10,000 to 100,000 times).
As stated above, conventionally only improvements in the heat, light or chemical resistance of lead chromate pigments during use have been attempted. Although lead chromate pigments have very low solubility, the dissolution of heavy metals, such as hexavalent chromium and lead, which dissolve in only slight amounts, has not been able to be sufficiently suppressed, so that improvements in the safety upon use of lead chromate pigments has not yet been successful.
With respect to negation of the toxicity of hexavalent chromium in a biological test by the addition of S-9 mix etc., Fernando L. Petrilli and Silvio De Flora have reported in Mutation Research, 54 (1978), pp. 139-147.
This toxicity negation was ascertained by the present inventors through many microbiological tests in which S-9 mix was added as a metabolic activator, as in the above report, to lead chromate pigment samples and most of the resulting pigments were considered as negative in inducing reverse mutation.
However, although lead chromate pigments are caused to be negative in reverse mutation induction by the toxicity-negating action of a material such as asorbic acid or the mixed agent consisting of G6 PD and S-9 mix as shown in the report by De Flora et al., this cannot be regarded as a substantive property improvement.
Therefore, in industrial fields where lead chromate pigments are manufactured and used, there is a strong need for a lead chromate pigment which in itself is negative in inducing reverse mutation and is, hence, safe from an environmental hygiene standpoint and which can be produced by a rational industrial process.